SCH 4U ENERGY CHANGES AND RATES OF REACTION

The Energy of Physical, Chemical and Nuclear Reactions Thermodynamics - the study of energy and energy transfer Thermochemistry - the study of energy involved in chemical reactions

Energy (symbol: E – unit: J joules) Law of Conservation of Energy - total energy of the universe is constant - energy can neither be created nor destroyed Δ universe=0 - energy can be transferred from one substance to another - energy can be converted into various forms

System - part of the universe that is being studied and observed (reactants & products) Surroundings - everything else in the universe - part of the universe that is likely to be affected by energy changes in the system

Universe = System + Surroundings Δ universe = Δ system +Δ surrounding = 0 Any change in the system is accompanied by an equal and opposite change in the surroundings. Δ system =−Δ surrounding

Heat (symbol: Q – unit: J joules) - transfer of kinetic energy (E k ) spontaneously from warmer to cooler objects

Temperature (symbol: T – unit: K Kelvin, °C Celsius degrees) TK = T°C measure of the average kinetic energy of the particles that make up a substance or system Celsius scale – relative to water (0°C water melting point, 100°C water boiling point) Kelvin scale – absolute scale (O K is temp. when a substance has no kinetic energy)

Enthalpy (symbol: H – unit: J joules) - total internal energy of a substance at a constant pressure - hard to measure; includes (1) energy of physical state, (2) energy in bonds, (3) energy in nucleus; includes: - moving electrons within atoms - vibration of atoms connected by chemical bonds - rotation and translation of molecules - nuclear potential energy of protons and neutrons in atomic nuclei - electronic potential energy of atoms connected by chemical bonds

- easily measure and study change in enthalpy (ΔH) in a reaction - enthalpy change of a process is equivalent to its heat change at constant pressure -chemical bonds are sources of stored energy -breaking a bond is a process that requires energy - creating a bond is a process that releases energy

Endothermic Reaction: - system absorbs E - more energy needed to break bonds than released by creating bonds - change in enthalpy is positive (+)

Exothermic Reaction : - system releases E - more energy released by creating bonds than needed to break bonds - change in enthalpy is negative (-)

Representing Enthalpy Changes ΔH rxn enthalpy of reaction (depends on temperature and pressure) ΔH° rxn standard enthalpy of reaction at SATP (25°C, 100 kPa)

(1) Thermochemical Equation – balanced equation that indicates enthalpy change

(2) Separate Expression

(3) Enthalpy Diagram

1.Physical changes - Small change in enthalpy (tens of kJ/mol) - Change in intermolecular forces between molecules as they change phase o ΔH vap enthalpy of vaporization (liquid gas) o ΔH cond enthalpy of condensation (gas liquid) o ΔH melt enthalpy of melting (solid liquid) o ΔH fre enthalpy of freezing (liquid solid) o ΔH soln enthalpy of solution (solid aqueous – dissolving)

2. Chemical changes - Moderate change in enthalpy (hundreds of kJ/mol) - Change in intramolecular bonds within molecules as reactants break apart to form products o ΔH comb enthalpy of combustion o ΔH neut enthalpy of neutralization o ΔH f enthalpy of formation

3. Nuclear changes - Enormous change in enthalpy (billions of kJ/mol) - Change in nuclear binding energy holding together the nucleus of an atom o A significant amount of mass of the reactants is actually converted to energy o Einstein: E=mc 2, where m=mass and c=speed of light (3.0x10 8 m/s)  so a tiny mass is equivalent to a significant amount of energy

Mass Defect  o difference in mass between a nucleus and its nucleons (particles found in the nucleus)  o caused by the energy associated with the strong force that holds a nucleus together  o the higher the nuclear binding energy, the more stable the nucleus Nucleus + Nuclear binding energy Nucleons